I posted this to the plumbing and heating group as well:

I'm rehabbing a bathroom in my old victorian rowhouse in Philly, and I
have a question about routing the drains for the tub and sink.
Currently they drain into the cast-iron toilet bend which has two
inlets on one side of it. The bathroom is on the second floor (of
three) over the top of another apartment. I would like to change the
configuration of things in the bathroom and route the drain pipes for
the sink and tub down an existing chase in the wall directly to the
basement where they would enter the main stack where it has been
replaced with PVC. I don't have an easy way to route a vent for those
two drains, however, so I'm wondering if I can drain to the stack in
the basement but vent through the inlets to the toilet bend. Is this a
totally non-compliant configuration? What sort of trouble would I have
doing this? Any advice would be appreciated.

Also, I would like to move the toilet 90 degrees. Since the current
cast iron ring has just two notches for mounting bolts, I want to
replace it. But I don't have any experience working with cast iron.
Is it just soldered on there? Can I heat it up, pop it off and stick
another one on there? Thanks,

Andrew